(1) Field of the Invention
The present invention relates to luminance and chrominance signal separating apparatus for separating a luminance signal and a chrominance signal from a composite color television signal.
(2) Description of the Prior Art
Recently, progress of digital signal processing technique accompanies that an adaptive type luminance and chrominance signal separating apparatus which selects a plurality of filters according to exsistence of correlation between an arbitrary reference signal and the signals at the vicinity of the position of the reference signal in a horizontal or vertical direction on the screen in luminance and chrominance signal separating apparatus which separates a luminance signal and a chrominance signal from a composite color television signal.
FIG. 1 is a block diagram of a luminance and chrominance signal separating apparatus in accordance with the prior art.
A bandpass filter 501 extracts a chrominance signal component from a composite color television signal (hereafter, called VBS signal, which means video burst signal). Each one of 1 H delay devices 502, 503 and 504 delays a signal by 1 H period, where 1 H means one horizontal period. In digital circuits, memory devices are usually used as delay devices. A comb filter 505 separates a chrominance signal from the output signal of the bandpass filter 501 and the output signal of the 1 H delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1 H period. A comb filter 506 separates a chrominance signal from the output signal of the 1 H delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1 H period and the output signal of the 1 H delay device 503 which is delayed from the output signal of the bandpass filter 501 by 2 H periods.
An adder 507 adds the output signal of the bandpass filter 501 and the output signal of the delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1 H period. An adder 508 adds the output signal of the 1 H delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1 H period and the output signal of the 1 H delay device 503 which is delayed from the output signal of the bandpass filter 501 by 2 H periods.
Absolute value circuits 509 and 510 calculate absolute values of the output signals of the adders 508 and 507, respectively. Low pass filters 511 and 512 filter the output signals of the absolute value circuits 509 and 510, respectively and output their average values. A comparator 513 compares the output signals of the low pass filters 511 and 512 and judges which output signal is smaller. A smaller average value means stronger correlation. A selector 514 selects an output signal of the comb filters 5 or 6 which has stronger correlation, based on the comparison result of the comparator 513. A subtracter 515 outputs a perfect luminance signal by subtracting a chrominance signal which is an output signal of the selector 514 from the signal which is delayed by 1 H period from the input VBS signal. A bandpass filter 516 outputs a perfect chrominance signal by limiting the bandwidth of the output signal of the selector 514.
A chrominance signal is extracted from the input VBS signal at the bandpass filter 501. The extacted chrominance signal component is supplied to the delay devices 502 and 503 in order and one input signal to the delay device 502 and the two output signals from the deley device 502 and 503 are supplied to the two comb filter 505 and 506. Because each comb filter 505 and 506 is supplied with two signals which have an 1 H period time difference, the output of each comb filter becomes a chrominance signal. A correlation value is obtained by adding an input signal and an output signal of each delay device 502 and 503 over an 1 H period. Because a chrominance signal inverts its phase every 1 H period, if the sum of the chrominance signal component during an 1 H period is zero, the chrominance signal can be regarded to have correlation. Actually, the average value of the absolute values of the sums during an 1 H period is a correlation value.
The comparator 513 compares the correlation values between an arbitrary reference signal and the signal 1 H ahead of the reference signal and between the reference signal and the signal 1 H behind the reference signal and judges which correlation value is smaller. The output signal of the comb filter 505 or 506 which is judged to have stronger correlation is selected at the selector 514 according to the output of the comparator 513. A luminance signal can be obtained by subtracting the chrominance signal selected at the selector 514 from the signal delayed by 1 H from the input VBS signal at the 1 H delay device 504. A better chrominance signal is obtained by passing the chrominance signal selected at the selector 514 through the bandpass filter 516.
However, a high frequency component of the luminance signal can not be distinguished with the chrominance signal in the above circuit configuration, misjudgment occurs in correlation judgment, for example, at the vicinity of the vertical edge of a multiburst signal and cross color interference occurs which a luminance signal mixes in a chrominance signal. A chrominance signal inverts its phase every 1 H period but the high frequency component of a luminance signal keeps the same phase. Therefore, if the high frequency component of the luminance signal has some correlation with the chrominance signal, the sum signal for the 1 H period is zero.
But if there is correlation in a luminance signal, the signal component is not zero but is double of the original component. Therefore, the correlation value differs depending on that it is correlation of a chrominance signal or correlation of a luminance signal.